Grids of LVG models for a range of molecular hydrogen densities and kinetic temperatures. Each grid corresponds to a different value of X_CO/(dV/dR), from left to right: 10^-6, 10^-7 and 10^-8 pc (km s^-1)^-1. The CO(1–0) radiative temperature is represented in color scale and is overlaid with R₂₁ black contours (0.3 to 1.3 in steps of 0.2) and R₃₂ blue contours (0.1 to 0.6 in steps of 0.1). The line ratios increase from bottom left to top right.